Microwave antenna array



April 0, 1954 w. B. HEBENSTREIT 'MICROWAVE ANTENNA ARRAY 2 Sheets-Sheet 1 Filed June 3, 1950 INVENTOR.

1,111,, 'IIIIIIIIIIIIA 'IIIIIIIIIIIIIIIIIIII v! kt/LL/AM B. HEBENSTRE/T April 20, 1954 w. B. HEBENSTREIT MICROWAVE ANTENNA ARRAY Filed June a,' 1950 2 Sheets-Sheet 2 -Iv W H v INVENTOR,

WILL/AM B. HEBENS TRE/T v BY 7M1. ygw

Patented Apr. 20, 1954 UNITED "STATES OFFI-CE MICROWAVE AN TENN A": ARRAY William B. Hebenstreit,,. Palos .Verdes Estates,

.,1 Cant, assignor to Hughes T001 CorppanyglHouston, Tex av corporation ,of Delaware .rApplication June 3, 1950;:Ser-i'aLNo. 166, 0

'- 1 This invention relates toantenna systems, and i :m0i'e :=particulai!1y=.;to antenna systems employ- -.ti;ng waveguides of helicalconfigurations.

The antenna system of the type-here. consteinplated comprisesa number of :radiator ele.

sments. whicharespaced; along: and; coupled to a transmission line, the transmission line-being 1most:conveniently provided in the; formpof a waveguide. Microwave. energy applied-to the i transmission-line isico upl-edito the array of radi-1 ator'elementsanclrr-adiatedtherefrom in a di rectional pattern, thepartieularshape and charactcristics oiirthee pattern being dependent upon "'he;;physica1-.=,Spacing and electrical-mhasing of filfifidifltOl selements z and-ppon. the configuration:

of the array formed by said radiator elements. .-A: scanning;-:action-;of this pattern {through a spatial angle may be obtained by varying the electricalephasingn of ,the :radiator elements.

in; order :to; provide: a-better understanding of he; inventionrandi of thezmanner, in; which its z -advantages; and -;;i,mproved characteristics are achieved,=-c,ertain .basicconsiderations of impor 451781106 inwaveguides andzwaveguide antenna arrays are first: described.

-Thephaservelocityat which energy is propaga,ted in a' hollow pipe Waveguide .is a variable 1.:quantityaz'dependent both upon ;the waveguide ,crosssectienal dimension: and upon ;the-' freaquency v:ofr the energy. Expressed; in another I manners involving-wavelength, wit; :may; be said that while. the-iree-space wavelength is dependaentxonly upon :frequencysthe wavelength of microwaveenergy as measured within at hOllOWzPlpe waveguide; ;is -;f urther: dependent .upon. a crossssectionahdimensiom of the waveguide andunder :..-practical conditions is always much greater than ;the:',.free-.space-.1 wavelength. For. a rectangular "waveguide, operating in thewdominant mode; "the relationship. between; guideewavelength .A and .1zfiree=spaceawavelength x given. by vthe -.equation ust beggiieatenthanqone-half of a seven-tenths oi a .freeespace wavelength. at the nominal operating. frequency.

'Waveguide antennas must themselves. satisfy several major. requirements to, yield apractical array, those of immediateimportancefor. present consideration being .(for .broadsideor CIOSSrfiIG .-direction ality) first, that the radiator: elements fedirom. the waveguide should have a. spacing along the line of thearray not. greaterthan approximately vtl'iree-foui'ths ,of .a freer-space wavelength; and --second, that the waveguide structure must be so proportioned andthe array elements sospaced-relative thereto raswto cause the, said elements to radiate in phase agreement, at the nominal operating frequency.

It might at first appear/tube impossible to .design a waveguide antenna satisfying thepracdeal-requirements mentionedvabove for in-phase excitation of the-radiator elements normally re- ,quires that the :spacing of these elementsrelative to the waveguide shall bea full guide-wave- ..length, generally obtainable only at the-expense of a physical spacing along the guide of very considerably greater than ,the recommended maximum (three-fourths free-space wavelength) mentioned above. -As one solutionto this-problem however, and now familiar to those versed in;this art, in-phase. radiation canrbe achieved r-with; aone-half guide-wavelength spacing of radiator elements, with a concurrent ireduction of their free-sspace-interval to a practicalvalue, by-reversing the coupling of alternateelements.

, For example, in \the -instancewhere the radiator elements are provided as apertures;in--the nar- 4. rower wall of *the.-waveguide,"this-reversal of coupling can beaccomplishedby makingdhe :apertures of elongated configuration, and; hav- -;ing themextend directions generallylcross- "wise, to ,the. axis of the waveguide in ,such manner that adjacentapertures are oppositely inclined relative to the said axis.

In; accordance .with the above-menticned ex- 2 pedient 'of coupling reversal, an advanced=;.=but -;otherwise now conventional design {01: a: crossfire microwaveantenna; here mentioneclior .com-

parison; purposes, calls for a lengtlr. of: hollow pipe waveguide having. a series of .elongatedaper yturesor slots .formed in awall. oi the. guidesand consecutively spaced therealong at substantially one-half guideewavelength intervals.

Thel dimansion b, of the waveguide-isso chosenthat, at nominal or 'CI'OSSefiIB operating frequency f0,

the guide-wavelength A o (corresponding to the cross-fire frequency) is-approxim'ately 1.5- times thefreespace wavelength; this occurs fora-value of b which is approximately two-thirds of the free-space wavelength. The resultant physical spacing of the slots along the waveguide is approximately three-fourths of a free-space wavelength, a value in agreement with that already indicated as satisfactory from a practical standpoint. It may be noted at this point that in order to overcome certain excitation difiiculties that often arise in cross-fire operation, an inphase radiation condition may be slightly departed from, causing the antennas directivity axis to extend just a bit askew to the cross-fire direction. Further, the waveguide parameters and operational frequency may also for other reasons be so chosen as to increase the angle between the directivity axis and the cross-fire direction to any desired value within certain limits dictated by waveguide transmission characteristics.

A conventional slotted waveguide antenna array as just described is quite satisfactory for radiating microwave energy in a fixed direction, relative to the line of the array, but many practical applications call for a scanning action of the energy. One of the possible methods of providing this scanning action is to frequencymodulate the excitation ener y, which causes variation in phase velocity and guide-wavelength and correspondingly produces variable phasing of adjacent elements. Such a method has been little used, however, principally for the reason that with waveguide antenna structures of conventional type such as that above referred to, the frequency excursion required to scan over a sector of, sixty degrees, i considerably larger than can be conveniently obtained from present day high-level microwave energy sources. Further, even if the problems imposed by this inadequacy can be overcome by improvements in microwave sources, the scanning action of a conventional antenna would have to be limited to sectors of considerably less than sixty degrees, in order to keep the effective spacing of radiating elements within satisfactory limits. This frequency modulation method of scanning neverthelcss would otherwise offerdistinct advantages, such as avoiding mechanical complexities presented by other types of scanning antenna systems, or reducing the space requirements needed f or their mounting.

It is therefore an object of the present invention to provide a novel type of apertured waveguide antenna structure having an improved frequency-shiit-scann'mg characteristic, that is, one which requires a relatively small frequency excursion to yield wide angle scanning action.

It is a further object of the present invention to provide an apertured waveguide antenna having a novel structure which offers a greater range of design control as to the effective electrical spacing between adjacent apertures.

The foregoing objects, and other objects, features and advantages of the invention which will appear from the description and from the claims appended hereto, are attained by providing an antenna array in which the transmission line interconnecting and feeding the radiating elements is given a helical conformation to produce an effective phase velocity along the line of the array which is a. small fraction of the corresponding free-space velocity of the microwave energy. In such a helical structure, the electrical phasing of adjacent radiating elements is no l on'ger limited by the ordinarily compromising effect of their required physical spacing, but may be made to have any desired value by suitable'choice of helical transmission line parameters. One of the important advantages stemming from this improved structure is that, while providing any desired element spacing as expressed in terms of iree-space wavelength, the electrical spacing may be increased to several transmission line wavelengths. Thus, in frequency-shift scanning the frequency excursion required to effect scanning action over a sector of specified angle is substantially reduced.

The invention will be more fully understood by reference to the following detailed description accompanied by the drawing in which:

Fig. 1 is a perspective view, partly in section, of a simplified helical waveguide structure illustrative of the present invention;

Fig. 2 shows another helical waveguide antenna structure embodying the invention;

Fig. 3 is a partial section of the above embodim rit taken along the line 33 of Fig. 2;

Fig. 4 is an enlarged view of a portion of the Fig. 2 waveguide structure encompassing a slot radiator;

Fig. 5 is a sectional view taken through the helical waveguide structure along the line 5-5 of Fig. l;

Fig. 6 shows another practical embodiment of a helical waveguide antenna;

Figs. 7, 8, and 9 illustrate a helical waveguide structure in several phases of its production by an electroforming process;

Fig. 10 represents a helical waveguide antenna embodiment including mechanical means for producing a scanning action of the directivity characteristic; and

Fig. 11 is a sectional view taken on line i i-l I of Fi 19.

Referring first to the perspective view of a simplified embodiment of the invention as illustrated in Fig. 1, there is shown at 512 a hollow rectangular waveguide having a helical conformation. A series of apertures it are formed in the outer wall or" the coiled waveguide, and are positioned to lie in a straight line it paralleling axis is of the helix. As illustrated, it is preferably the narrow wall 2% of the waveguide which is faced outwardly to form the outermost surface of the helix. By constructing the helix in this manner, the center-to-center distance d between adjacent apertures, corresponding to the pitch of the helix as measured along aperture line it, may be made equal to or somewhat less than three-fourths of a iree-space wave-length as is desirable for optimum pattern characteristics in cross-fire or near cross-iire operation. It may also be here noted with respect to Fig. 1 that the effective length of waveguide which interconnects a pair of adjacent apertures, best determined empirically, may be taken as equal to the length S between adjacent apertures along the center line of the waveguide. This length S is sufficiently close approximation to the effective wave-guide length for present purposes.

As already indicated in the preceding general observations of waveguide antenna considerations, the antennas directivity axis extends at a right angle to the aperture line is when there is in-phase radiation from apertures M. Using circular apertures centered in the narrow wall of the waveguide as shown, or using any other equivalent type of apertures or radiator elements, this condition is obtained when the eiiective length S of the waveguide portion interconnecting a pair of adjacent aperturesis an integral multiple -of a fiill'guide-wavelength at-cross-"fire frequencyi A"-'d'eparture-- from this cross-fire frequency results vin 1 a -change ofrelative phasing between apertures and causes the directivity axis 7 22 to extend at an angle ,0 relative to th'e crossfile direction: 24, asmeasured theplane deby aperture'line l 6 and helixaxis -|'8'.

The manner in =which the angle-yids dependent uponope-rational frequency may be further explained and specificallyderived asfollows-:

A' uniph ase-front of energy radiated- 'by-the-antenna isg in- Fig.1, indicated by line 26- extending fromarr aperture A; at a right angle to the directivity -line 22 which extends fromeana'djoining v aperture--13, and intersecting directivi'tyline Hat/point 0. The-phase shift as measured from B to'G is given bythe-equation =21r% Sill 'tllwhere n is:the free-space iwavelengtlr atithe operating frequency; f; a. The phase:rshifttwaimthe waveguide 'betweenozthepoints B; and A is:--given bymhe equation:

, where; hgxiS. the guide wavelength corresponding Aninspection of Equation 6" makes'readily apparent cei=tain= of the improved capabilities achieved by the present invention. For'awaveguide ofi given cross-sectional dimensions; the magnitude of theexpression is; controlled :by the frequency Y excursion; For" a fixed frequencyexcursion, then, the 'magnitud of scan angled/is a direct :function of "the ratio S,

For: scanning action-over a given ilarge angle 0f say-psixty; degreesythe: frequency excursion requi-red is reducedbyapproximatelylthe multiplying'factor i and by verycloselythat factor-for small. scan angles; This maybe made-more evident :byplacing Equation 6 in a formusing-frequency:terms, in theiollowing manner; The-factorflbin Equation. 1 given above is'equalaito-the cut-off'wavelength kc, and. the ratio is exactly equal to the ratio where fc represents the cut-ofi frequencyl Equaltion' -'1 may 'thus be equivalently expressed as" Substituting (7) into '(6) where jo'represents'the cross-fire frequency. Defining 'K: as: the ratio of the vfrequency excursion toetheccross fire frequency; so that f=fo(*1+K) substituting iii-Equation 1; andvexpanding l in powers-o-f K:

K f., Toner-ma 1 For. small angles, then, it is. sufiiciently. accurate tontake-flintomaccount.only the first term..to the right of the equalitysign in Equation 9, ;so that, where 41/ is expressed in degrees,

of the general :type? here under 'consideration,= so

that-relative to the conventionalantennadstructur'e" earlier described; the multiplying factor is equal to the reduction:irrirequency excursion ratime whiletthe preceding description of certain basic principles; of 1 the invention has been given with reference l to'zthe: essentiallyJ'diagrammatic I illustratiorr in'=Eig.; 1, it is to be understood that various techniques andaaexpedients which are conventional in the antenna art may besemployed therewith-s For:examplethefrequencyshift sys- 1 tern of soanningrrequires that the standing wave ratio in thefeed: transmission line be small. This maybe: accomplishedaby providing aproper termination ofv the helical transmission line.v As another "example," in a relatively longarray as mustabe::nsed:to.iproduce a very sharply directional: .characteristic,- the coupling of consecutive radiator velem-ents. must generally be of progressivelygreater magnitude; in going along; the array toward the-termination, inrorder to provide :a.; uniformly; illuminated array, Again, where a minimum side lobe :level .is desired for a .given width: OfLthel major lobe," certainnon unifrorm illuminations are provided in a manner familiar to those versed in the art.

Figsev2I-tandl3 illustrate another practical embodiment of the present invention, in'which there isalsoiprovided .a progressive variation in radiator.- element coupling by; employing suitablyprientedvelongated apertures in the narrow wall .of

the: waveguide.

The waveguide is here formed by machi-ning, from a: metal cylinderya rec- 'tangu-lar thread screw 28' of suitable pitch,

andeshrinking; a-metal tube 3E over: th machined screw-to produce a waveguide" hav- 701' ingv a helical axis and providin a helical transmission path. The waveguide thus: producedzhas-auniform rectangular cross-section :in all planesrperpendi'cular to its helical axis. This structure may? be fabricated "01: aluminum; for

I example;:.;and where-airing :arra'yl is desired the screw may bemade by a centerless g i method. A shrink fit is obtained by reaming or hOIilIlg the tube (it to an inside diameter slightly ameter will be increased sufficiently to permitinsertion of the machined screw 28 therein. Dumbbell shaped slots 32 are out through the outer wall of the helical waveguide to serve as the radiator elements, it being understood that dipole or other types of radiators may be alternatively utilized. Conventional COllDllIlg structures 34 and 36 are provided at the ends of the helical waveguide antenna, to serve as means for connecting to a source of microwave energy, and to an absorptive load for providin a refiectionless termination.

The dumbbell shaped slots 32 are so proportioned that their perimeters are each substantially equal to a full waveguide wavelength. Positioned in the rectangular waveguides narrower Wall in a manner as here illustrated, the slots interrupt and are excited by the flow of surface currents in the waveguide which are associated with the dominant mode of wave transmission, and radiate quite effectively because of their resonant dimensioning. Referring to the Fig. 4 enlarged view of a typical slot region in the disclosed waveguide structure, the coupling of a slot radiator 32 is dependent upon the magnitude of its inclination 6 relative to the cross-Wise direction 38 in the waveguide passage at that point. Thus, any desired illumination distribution along the array may be obtained by a progressive variation of slot inclinations in some suitable manner, as generally indicated in Fig. 2.

In a design of an antenna array utilizin slot radiators in the narrow wall, the length of a resonant slot radiator may be greater than the narrow wall dimension, as illustrated in Fig. 5. In order to accommodate such a slot and still have it match properly to the waveguide and radiate effectively, the ends of the slot are given access to the interior of the waveguide by removing wall material adjacent thereto, as indicated at le and 42, to a depth of substantially one-fourth wavelength at nominal frequency. where it is desired to make length S a multiple of half a guide-wavelength, rather than a multiple of a full guide-wavelength as thus far described, the inclinations of adjacent slots must be in opposite directions relative to the crosswise directicn in the waveguide, in order to provide coupling reversal as already described.

Another method of fabricating th helical Y waveguide is illustrated in Fig. 6. Here again a rectangularly threaded screw 50 is machined from a metal cylinder, but in this instance a shoulder 52 is provided on the projecting helical rib 54 of the screw. The fourth wall enclosing the waveguide is formed by an elongated metal strip 56 which is wrapped around the threaded core, strip 56 having a width suitable to fit snugly into the shallow passage defined by the configuration of the shouldered rib. Outer wrapping Eli-may be bonded to core 59 by soldering, brazing, welding or any other method suitable to the materials employed and capable of providing good electrical continuity. For this purpose, the core and strip may be made of a material inherently suitable to accept such banding treatment, or may be plated or otherwise prepared to accept such treatment. For example, the core and strip may be fabricated of an aluminum alloy, in the interests of both strength and lightness, then plated with copper or additionally with silver,

rendering the core and wrapping suitable for bonding by one of the soldering processes.

Still another method of fabricating the helical waveguide structure is shown in Figs. 7, 8. and 9, wherein a screw core 66 is provided as already described with reference to Fig. 2. Core 86 may in this instance be made of any desired material, for it here functions primarily as a base upon which is plated a conductive surface 52, and upon which an outer wall 5:; is then provided, as by the lost wax process of electroforming. For example, core 58 may be fabricated of steel, and the plating applied thereto may be of nickel. The cavity formed by the helical thread may then be filled with aluminum, as indicated at 65, or with any one of the commercially available alloys 01' waxes specifically formulated for use in the lost wax process, these materials being characterized by low melting-points but nevertheless machinable at ordinary temperatures. The excess of aluminum, wax, or alloy filling should then, of course, be machined away to present a perfectly true cylindrical surface which is in part formed by the plating 62. Plugs or cores 658 having a desired configuration of radiator apertures 20 are then fastened, as by screws '52, in their proper positions against the composite cylinder, and the resultant assembly is again plated with nickel to form a relatively thick outer wall 64. The salient portions of plugs 63 and their fastening elements are then machined off to again provide a smooth cylindrical surface, and the cavity-filling material is removed in some suitable manner. For example, in the specific structure here described, the aluminum may be removed by treatment with a hydrochloric acid solution which attacks the aluminum without appreciable reaction upon the nickel plating. Where low melting-point fillers are used, their removal may be effected simply by a heating process. Whatever the specific material and filler-removal method used may be, however, the end result is the production or a helical waveguide havin essentially continuous walls formed of homogeneous conductive material. Alternatively, the radiator apertures '39 during the electroforming process, may be machined in the desired locations after the outer wall of the cylindrical structure has been formed. Where greater electrical conductivity is desired than is provided by the nickel or other material with which the walls may be surfaced, a final plating of silver or other highly conductive material may be applied.

While the waveguide structures thus far disclosed make use of a single line of radiator elements which yield a radiation pattern narrow in a plane parallel to the line of the array but [broad in a plane perpendicular thereto, it is to be understood that the pattern in the latter plane may be modified in any desired manner by using additional radiator elements, or by using the helical waveguide antenna as a source of radiation to be shaped by reflectors or baffles or other such means as are conventional, or by using each of the radiator elements to excite supplementar arrays extending in planes perpendicular to the helix axis. It is also to be understood that the coiled waveguide arrays here disclosed are not limited to scanning by the frequency modulation method. It may in some instances be desirable to provide a waveguide antenna array, of the sinuate type here disclosed, in which scanning action may be effected by mechanical means. Various techniques or modifications employed with other waveguide antennas are available. As examples, the antenna array may be designed for fixed frequency operation and bodily rotated to produce a scanning action, or the helical waveguide structure may be provided with supplementary velocity-changing members penetrating the waveguide to a variable extent, or variably positioned therein, these being merely applications of conventional expedients. Still another method which may be employed is illustrated in Fig. 10. Here the efiective length S of the convolutions is itself made variable by the use of corrugated or bellows type of Waveguide sections M which may be repetitively lengthened and shortened. These sections are provided at opposite sides of each convolution, and by way of example, the necessary mechanical movements may be imparted by a rack and gear arrangement as here shown. Members 16 and 18 are structurally rigid bars which extend along the waveguide structure and interiorly thereof, secured to each convolution. At as many points along the helical structure as may be necessary to lend rigidity thereto and uniformity of incremental changes in the spiral length S therealong, are provided spacing members 80 which engage the bars 15, 18 in such manner that the lengths S may be controlled by threading action of the spacing members. Such threading action may be readily achieved by providing a rack 82 in engagement with gears 84 mounted upon the said spacing members 80. Reciprocative movements of rack 82 thus effect a variation in the length S 01' convolutions interconnecting adjacent radiator elements, resulting in variably phasing the radiator elements as is necessary to produce scanning action.

The above-described structures are illustrative of principles of the present invention. It should be apparent that modifications and other arrangements will readily occur to those versed in the art without departing from the scope or spirit of the invention defined in the appended claims.

It is also to be understood that, in addition to their major applications as transmitting devices, the helical transmission line antenna arrays forming the subject matter of the present invention may also be useful in certain applications as receiving devices, their characteristics in such cases being exactly analogous to those pertaining to transmission.

What is claimed is:

1. An antenna array for radiating microwave electromagnetic energy comprising: a tubular element, a cylindrical member, said cylindrical member being provided with external substantially rectangular threads and being concentric with said tubular element, said tubular element [being in contact with the threads of said cylindrical member to define a duct of helical configuration and 01 substantially rectangular cross 10 section, and said tubular element having a plurality of openings spaced apart to communicate with said duct whereby said duct operates as a waveguide for microwave electromagnetic energy supplied thereto and radiated from said openings.

2. An antenna array for radiating microwave electromagnetic energy comprising: a conductive tube, a rectangular thread screw, said screw being concentric with said tube and in contact therewith to provide a duct having a substantially rectangular cross section to define a wave guide of helical configuration, and said tube being provided with a plurality of slots communieating with said waveguide for radiating microwave electromagnetic energy therefrom.

3. An antenna array for radiating microwave electromagnetic energy of a predetermined frequency comprising: a metallic tube having a plurality of uniformly spaced slots which are axially aligned relative to each other, a metallic screw having a rectangular thread, said screw being concentric with and in contact with said tube, whereby said screw and said tube cooperate to define a duct of substantially rectangular cross section and extending axially in the form of a. helix, said slots being spaced apart so that alternate ones of said slots communicate with alternate turns of said helical duct, said duct constituting a waveguide for microwave electromagnetic energy, and said slots radiating said energy from said waveguide in a direction determined by said predetermined frequency.

4. An antenna array according to claim 3 3 wherein said slots have a spacing not greater than three-quarter wavelengths in free space, and approximately equal to an integral number of wavelengths in said waveguide, at said predetermined frequency.

5. An antenna array for radiating microwave electromagnetic energy comprising a hollow cylinder having an inner surface, a cylindrical mem- :ber in concentric relationship with said element, said member having external substantially rectangular threads in contact with said inner surface, whereby said threads and said inner surface form a hollow duct of helical configuration and of substantially rectangular cross section, and a plurality of openings in said hollow cylinder positioned to communicate with the respective portions of said duct.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,408,435 Mason Oct. 1,1946 2,438,735 Alexanderson Mar. 30, 1948 2,526,573 Mason Oct. 17, 1950 OTHER REFERENCES Micro-Waveguides. by Virginia Walters, Radio-Electronics, Dec. 1948, pages 24 to 26, 

